A variety of in vivo gene therapy products currently under development are based on the delivery of a therapeutic transgene by recombinant viral vectors. A common vehicle for the delivery of transgenes are recombinant adenoviruses, usually those deficient for replication in any cell other than a specific packaging cell line. These packaging cell lines express certain adenoviral genes necessary for virus replication that have been deleted in the deficient virus. For the production of adenoviruses containing deletions in the E1 region, the most commonly used cell line is the 293 cell line. The production of replication deficient adenovirus in 293 cells is difficult because the cell line is difficult to grow. For example, 293 cells require attachment to a substrate and appear to differentiate at high confluency. Another limitation is that replication deficient adenoviruses do not replicate as well as wild type viruses. While the specific virus production for wild type adenoviruses in 293 cells is approximately 80,000-100,000 particles per cell, E1 replication deficient adenoviruses typically produce only 100-2000 particles per cell. Based on current assessments of dosage regimens and therapeutic market size, estimates have indicated that annual production of approximately 10.sup.18 particles will be necessary to satisfy demand for some gene therapy products. Improvements in the production of recombinant adenoviruses to levels which will satisfy the anticipated market for adenoviral gene therapy products is therefore required to make this technology commercially feasible.
The present invention describes a microcarrier based process for the production of viral vectors in anchorage dependent packaging cell lines, which allows for cost-effective production of adenoviral gene therapy products sufficient to meet the projected market demand. The invention describes a scaleable production process which produces greater than 2.times.10.sup.15 viral particles in a 5 liter bioreactor. This process is fully scaleable to achieve the projected 10.sup.18 particles per year with a bioreactor as small as 100 liters and 5 liter purification columns.